Photocatalysts made by using oxides containing metal ions of d10 electronic state

ABSTRACT

A photo-catalyst comprising RuO 2 -loaded A 2 X n O m  or RuO 2 -loaded BX n O m  (wherein A is an alkali metal atom, B is an alkali earth metal atom or Zn, X is a metal ion with d 10  configuration, n=m/2 or n=m/3 and n is 2), and especially relates to a photo-catalyst for complete photo decomposition reaction of water.

FIELD OF THE INVENTION

The present invention relates to a novel photo-catalyst. Especially, thepresent invention relates to a photo-catalyst comprising RuO₂-loadedA₂X_(n)O_(m) or RuO₂-loaded BX_(n)O_(m) (wherein A is an alkali metalatom, B is an alkali earth metal atom or Zn, X is a metallic ion withd¹⁰ configuration, n=m/2 or n=m/3 and n is 2), more especially relatesto a photo-catalyst used for complete decomposition of water.

BACKGROUND OF THE INVENTION

The photo decomposition reaction of water is broadly interested from theview point of photo energy transformation. While, a photo-catalyst whichshows activity to the photo decomposition reaction of water can beconsidered to be a high quality photo functional material possessingfunctions such as photo absorption, electric charge separation orsurface oxidation-reduction reaction.

Kudo, Kato et al are explaining that alkali tantalate or alkali earthelements are the photo-catalyst showing high activity to the completephoto decomposition reaction of water by quoting various prior arts[Catal. Lett., 58(1999). 153-155, Chem. Lett., (1999), 1207, SurfaceVol.36, No.12(1998), 625-645 (shortened to document A)].

In above mentioned document A, there is an explanation about an usefulphoto-catalyst materials for proceeding the decomposition reaction ofwater to hydrogen and oxygen using a photo-catalyst, and manyindications about photo-catalyst used for complete photo decompositionreaction of water are mentioned.

In the document, it is reported that the authors has first at all found,that tantalate acts as a photo-catalyst which generates oxygen andhydrogen from pure water according to stoichiometric ratio without apromoter, and then has found that the catalyst activity is remarkablyimproved by carrying NiO promoter (right column of 635^(th) page).

Further, there is an explanation reporting that also K₄Nb₆O₁₇ cangenerate oxygen and hydrogen from pure water without a promoter, and thefine crystalline of it obtained by pulverizing it indicates remarkablyhigh activity. Furthermore, there is an illustration that a compoundhaving perovskite structure such as K₂La₂Ti₃O₁₀ has excellentphoto-catalyst activity, because the hydration reaction can easily occurbetween layers.

In FIG. 6 of 629^(th) page, there is an explanation about main elementscomposing of un-uniform type photo-catalyst materials, and in whichthere is a description explaining that the oxide composed of atransition metal ion possessing d⁰ and d¹⁰ electron arrangement or ametal ion of p-block. And, in 640^(th) page, following explanations aredescribed. Namely, the band gap of In₂O₃(ZnO)_(m) composed of laminatedstructure of In₂O and ZnO₃ is smaller than that of In₂O₃ or ZnO alone,and generates hydrogen by photo decomposition of methanol aqueoussolution under the visible radiation or generates oxygen by photodecomposition of AgNO₃ aqueous solution under the visible radiation.Further, regarding to β-Ga₂O₃, there is also following explanation.Namely, by forming a solid solution with In₂O₃, the band gap of itbecomes smaller maintaining the structure of β-Ga₂O₃, and can generatehydrogen and oxygen by photo decomposition of water under the presenceof methanol or AgNO₃ which are a sacrificial reagent, but not extends tothe responsibility for visible radiation. However, there is noexplanation teaching activity in regard to water decomposition reactionor Sb.

Concerning above mentioned circumstances, the inventors of the presentinvention have carried out intensive study to make the photo-catalystwhich uses a transition metal ion possessing d¹⁰ electron arrangementgenerate high activity, especially have repeatedly continued variousexperiments in regard to various compounds aiming to provide a catalystwhich make enable the photo decomposition of water.

From the results conduced by these experiments, the inventors of thepresent invention have found the following fact and have accomplishedthe present invention. That is, the salt of alkali metal or alkali earthmetal and transition metal ion possessing d¹⁰ electron arrangement canact as a catalyst which make enable the complete photo decomposition ofwater by loading RuO₂, without the presence of methanol or AgNO₃ whichare a sacrificial reagent used in above mentioned prior arts.

DISCLOSURE OF THE INVENTION

The present invention is the photo-catalyst comprising RuO₂-loadedA₂X_(n)O_(m) or RuO₂-loaded BX_(n)O_(m) (wherein A is an alkali metalatom, B is an alkali earth metal atom or Zn, X is a metal ion with d¹⁰configuration, n=m/2 or n=m/3 and n is 2). Desirably, A₂X_(n)O_(m) orBX_(n)O_(m) (wherein A is an alkali metal atom, B is an alkali earthmetal atom or Zn, X is a metal ion with d¹⁰ configuration, n=m/2 orn=m/3 and n is 2) is the photo-catalyst obtained by a coprecipitationpowder calcination method calcined at the temperature of 1150 to 1200°C., more desirably, X is In, Ga or Sb, an alkali metal atom is Na, K orRb and an alkali earth metal atom is Ca, Sr or Ba.

The second important point of the present invention is thephoto-catalyst for photo decomposition reaction of water comprisingRuO₂-loaded A₂X_(n)O_(m) or RuO₂-loaded BX_(n)O_(m) (wherein A is analkali metal atom, B is an alkali earth metal atom or Zn, X is a metalion with d¹⁰ configuration, n=m/2 or n=m/3 and n is 2).

BRIEF ILLUSTRATION OF DRAWINGS

FIG. 1 is the flow chart illustrating the preparation method of CaIn₂O₄at the preparing process of RuO₂-loaded CaIn₂O₄ photo-catalyst.

FIG. 2 is the graph showing the complete decomposition characteristic ofwater by RuO₂-loaded CaIn₂O₄ photo-catalyst.

FIG. 3 is the graph showing the difference of CaIn₂O₄ preparationmethods, a coprecipitation powder calcination method (a) and an oxidecalcination method (b), further showing the relationship with activityof catalyst when they are used for the preparation of photo-catalyst.

FIG. 4 is the graph showing the relationship between activity ofRuO₂loaded CaIn₂O₄ photo-catalyst at complete decomposition of water andloading concentration of Ru.

FIG. 5 is the graph showing the relationship of the calcinationtemperature and photo-catalyst activity at the preparation ofcoprecipitation powder calcined Caln₂O₄.

FIG. 6 is the graph showing the calcination temperature and thecharacteristics of diffusion reflection spectrum in the preparationmethod of Caln₂O₄.

FIG. 7 is the graph showing the complete water decompositioncharacteristics of 1 wt % RuO₂-loaded SrIn₂O₄ photo-catalyst.

FIG. 8 is the graph showing the relationship of the calcinationtemperature and photo-catalyst activity at the preparation of SrIn₂O₄.

FIG. 9 is the graph showing the calcination temperature and thecharacteristics of diffusion reflection spectrum in the preparationmethod of SrIn₂O₄.

FIG. 10 is the graph showing the difference of effects of alkali earthelements to the photo-catalyst activity in regard to RuO₂-loaded MIn₂O₄(M=Ca, Sr, Ba) photo-catalyst.

FIG. 11 is the graph showing the effects of alkali earth elements to thecharacteristics of diffusion reflection spectrum in regard to MIn₂O₄(m=Ca, Sr, Ba).

FIG. 12 is the graph showing the photo decomposition characteristics ofwater of RuO₂-loaded ZnGa₂O₄ photo-catalyst.

FIG. 13 is the graph showing the photo decomposition characteristics ofwater of RuO₂-loaded Na₂Sb₂O₆ photo-catalyst.

THE BEST EMBODIMENT TO CARRY OUT THE INVENTION

The present invention will be illustrated more in details. The methodfor preparation of indate of RuO₂-loaded alkali earth is describedhereinafter. The similar method can be applied for the preparation ofindate of alkali metal and Zn and for the preparation of Ga salt or Sbsalt of alkali metal, alkali earth metal and Zn.

I. Preparation Method of a Photo-Catalyst

a The preparation of alkali earth indate is; alkali earth metalcarbonate and indium oxide are mixed by prescribed molar ratio andcalcined by the temperature of 1000° C. in the atmosphere for 16 hours(called as a oxide calcination method), or,

b. In the ethanol solution containing alkali earth metal nitrate andindium nitrate by prescribed molar ratio, ethanol solution of oxalicacid is dropped and obtain white sediment. After said sediment is heatedat the temperature of 80° C., cooled down and filtrated then thefiltrated product is calcined by the temperature of 900-1300° C. for 16hours (coprecipitation powder calcination method) (the calcination timecan be adjusted concerning calcination temperature).

The flow chart showing CaIn₂O₄ preparation process by coprecipitationpowder calcination method is shown in FIG. 1.

In regard to RuO₂ loading, by penetrating method using THF(tetrahydrofurane) solution of Ru₃(CO)₁₂, which is a carbonyl complex,or aqueous solution of ruthenium chloride RuCl₃, ruthenium metal iscarried on indium salt so as the wt % of Ru in photo-catalyst to be 0.25to 2.0 wt % and make Ru to RuO₂ by heating it to 400° C., thus thephoto-catalyst is prepared.

II. The Measuring Method of Complete Water Decomposition Characteristicsof the Photo-Catalyst.

For the measurement of activity of photo-catalyst reaction, a closedcirculation type reaction apparatus composed of vacuum part, reactionpart and analysis part. Generated gases from photo completedecomposition reaction of water are circulated with Ar of 100 torrpreviously added to the reaction circulation apparatus during thereaction using a piston pump, and the generated gases are analyzed fromtime to time by a gas chromatography which is directly connected to thereaction part.

0.25 g of photo-catalyst powder is poured into a vertical reactor madeof quartz and suspended by pure water prepared by further deionizationof distilled water. The stirring of the photo-catalyst is carried out bybubbling of Ar of 100 torr pressure added in the reaction circulationapparatus. As the photo radiation, 500WXe lamp light (wave length range;from 260 nm to 600 nm) or Hg-Xe lamp light (wave length range; from 248nm to 436 nm) are used.

EXAMPLE 1

Photo-catalyst activity of RuO₂-loaded Caln₂O₄ RuO₂-loaded CaIn₂O₄photo-catalyst is prepared by calcined temperature of 1000-1300° C. Theobtained compound is analyzed by an X-ray diffraction method. The mainpeaks appear at 2θ=18°, 33°, 48° and 58°, and the highest peak appearsat 2θ=33°, which indicates a diffraction pattern properly correspondingto a diffraction pattern described in JSPD card (17-643).

The calcined temperature is changed to 1150° C. and 1 wt % of CaIn₂O₄carrying RuO₂is obtained. Then the obtained RuO₂-loaded CaIn₂O₄ is usedso that the activity of photo-catalyst and the change of the activitywith the passage of time is measured using a closed circulation typereaction apparatus described in paragraph II. Since a generation testapparatus is a closed system, gaseous hydrogen and oxygen aredischarged. Said discharging operation is repeated for three times, andthe change of activity is investigated. By the investigation, it becomesclear that the activity of catalyst maintains the initial feature after13 hours of total reaction time.

In FIG. 3, the activity of RuO₂-loaded CaIn₂O₄ photo-catalysts preparedby a coprecipitation powder calcination method (a) and an oxidecalcination method (b) are indicated in comparison. From this graph, itbecomes clear that the photo-catalysts prepared by a coprecipitationpowder calcination method (a) shows 14 times higher activity than thatof the photo-catalysts prepared by method (b).

In FIG. 4, the dependency of Ru concentration which effects to theactivity of photo-catalyst is indicated. The activity of photo-catalystincreases along with Ru concentration till 0.1 wt %, becomes almostconstant at 1 wt % and slightly decreases at 2 wt %. From thisphenomenon, it is understood that the optimum Ru concentration is 1 wt%.

FIG. 5 is the graph showing the relationship of the calcinationtemperature and photo-catalyst activity at the preparation of CaIn₂O₄.In a case of coprecipitation powder calcined CaIn₂O₄, when calcined bythe temperature higher than 900° C., activity is generated and increasesalong with the elevation of calcined temperature, and most high activitycan be obtained by calcined temperature from 1150° C. to 1200° C. Fromthese results, it becomes clear that the optimum temperature forcalcination is from 1150° C. to 1200° C.

In FIG. 6, the diffusion reflection UV spectrum of alkali earth indateprepared by altering the temperature for calcination from 1000° C. to1300° C. Photo absorption starts from approximately 450 nm, has a mildabsorption peak at 400 nm, a sharp absorption peak at shorter wavelength than 380 nm and indicates maximum absorption peak at 310 nm.

EXAMPLE 2

Photo-catalyst activity of RuO₂-loaded SrIn₂O₄ In an X-ray diffractionpattern of SrIn₂O₄ calcined by the temperature from 800° C. to 1200° C.,the main peaks appear at 2θ=18°, 33°, 35°, 47° and 58°, and the highestpeaks appear at 2θ=33° and 35° . Within this calcination temperaturelimit, a diffraction pattern corresponding to the diffraction patterndescribed in JSPD card (33-1337) is obtained.

In FIG. 7, the variation by aging of photo-catalyst of SrIn₂O₄ loading 1wt % of Ru by impregnation method using Ru carbonyl complex Ru₃(CO)₁₂ isindicated. Hydrogen and oxygen are generated. It is understood theinitial catalyst activity is maintained after total reaction time of 8hours.

FIG. 8 indicates the relationship between calcination temperature andphoto-catalyst activity at the preparation of SrIn₂O₄. Thephoto-catalyst activity increases along with the elevation ofcalcination temperature at the higher temperature than 800° C.,indicates maximum activity at 1000° C., and decreases at the highertemperature than 1000° C. That is, it is understood that the optimumcalcination temperature is 1000° C.

FIG. 9 indicates the diffusion reflection UV spectrum of SrIn₂O₄prepared by altering the calcination temperature from 1000° C. to 1300°C. Photo absorption starts from approximately 450 nm, has a mildabsorption peak at 400 nm, a sharp absorption peak at shorter wavelength than 380 nm and indicates maximum absorption peak at 310 nm.

EXAMPLE 3

Effect of M ion to RuO₂-loaded MIn₂O₄ (M=Ca, Sr, Ba) photo-catalyst FIG.10 indicates, activity of water photo decomposition of photo-catalystswhen the kind of alkali earth element is changed. In a case of Ba,activity can not be observed, on the contrary, in a case of Sr, activityis generated and further activity can be observed in a case of Ca.

FIG. 11 indicates the diffusion reflection UV spectrum of alkali earthindate MIn₂O₄ (M=Ca, Sr, Ba) when kind of alkali earth metal M ischanged. In cases of Sr and Ca, photo absorption starts fromapproximately 450 nm, has a mild absorption peak at 400 nm, a sharpabsorption peak at shorter wave length than 380 nm and indicates maximumabsorption peak at 310 nm.

In the meanwhile, in a case of Ba, a remarkable absorption peak isobserved at 440 nm, has a mild absorption peak at 400 nm and indicatesmaximum absorption peak at 350 nm.

EXAMPLE 4

Photo-catalyst activity of RuO₂-loaded MGa₂O₄ RuO₂-loaded ZnGa₂O₄ isprepared according to the preparation method of photo active catalystmentioned above. Water photo decomposition characteristics ofRuO₂-loaded ZnGa₂O₄ photo-catalyst is shown in FIG. 12.

It is understood that water is completely decomposed to hydrogen andoxygen by this photo-catalyst.

EXAMPLE 5

Photo-catalyst activity of antimonide salt RuO₂-loaded Na₂Sb₂O₆RuO₂-loaded Na₂Sb₂O₆ is prepared according to the preparation method ofphoto active catalyst mentioned above. Water photo decompositioncharacteristics of RuO₂-loaded Na₂Sb₂O₆ photo-catalyst is shown in FIG.13.

It is understood that water is completely decomposed to hydrogen andoxygen by this photo-catalyst.

Possibility for the Industrial Use

The photo-catalyst which carry out the complete photo decompositionreaction of water is usually limited to a metallic salt with d⁰configuration. In the present invention, it is found out that a metallicsalt with d¹⁰ configuration can be also used for the complete photodecomposition reaction of water. Therefore, it is obvious that thepresent invention contributes to the development of a novel catalyst forthe complete photo decomposition of water.

What is claimed is:
 1. A photo-catalyst comprising RuO₂-loadedA₂X_(n)O_(m) or RuO₂-loaded BX_(n)O_(m), wherein A is an alkali metalatom, B is an alkali earth metal atom or Zn, X is a metal ion with d¹⁰configuration, n=m/2 or n=m/3 and n is
 2. 2. The photo-catalyst of claim1, wherein RuO₂-loaded A₂X_(n)O_(m) or RuO₂-loaded BX_(n)O_(m) isobtained by a coprecipitation powder calcination method calcined attemperature of 1150 to 1200° C.
 3. The photo-catalyst according to claim2, wherein X is In, Ga or Sb, alkali metal atom is Na, K or Rb andalkali earth metal atom is Ca, Sr or Ba.
 4. The photo-catalyst accordingto claim 1, wherein X is In, Ga or Sb, alkali metal atom is Na, K or Rband alkali earth metal atom is Ca, Sr or Ba.
 5. A powder compositionyielding a complete photo-decomposition reaction of water comprising aphoto-catalyst comprising RuO₂-loaded A₂X_(n)O_(m) or RuO₂-loadedBX_(n)O_(m), wherein A is an alkali metal atom, B is an alkali earthmetal atom or Zn, X is a metal ion with d¹⁰ configuration, n=m/2 orn=m/3 and n is
 2. 6. The composition of claim 5 wherein thephoto-catalyst is obtained by a coprecipitation powder calcinationmethod calcined at a temperature of 1150 to 1200° C.
 7. The compositionof claim 5 wherein in the photo-catalyst X is In, Ga or Sb, alkali metalatom is Na, K or Rb and alkali earth metal atom is Ca, Sr or Ba.